The present invention relates to processes for alkylating isoparaffin hydrocarbons, such as isobutane, with olefin hydrocarbons, such as propylene and/or butylenes, in the presence of a sulfuric acid alkylation catalyst for production of alkylated hydrocarbons, wherein hydrocarbon effluent from an alkylation zone, comprising unreacted isoparaffin hydrocarbon and alkylated hydrocarbons and containing acid alkyl sulfates and dialkyl sulfates and acid oils, is flashed at reduced pressure in indirect heat exchange contact with reaction mixture in said alkylation zone for refrigerating said reaction mixture, and wherein flashed vapor and unflashed liquid from said effluent flashing step is fractionated to recover isoparaffin for recycle and alkylated hydrocarbon product. More particularly, the present invention relates to an improved process wherein hydrocarbon effluent, separated from alkylation catalyst, from the alkylation reaction zone is treated with bauxite or similar adsorbents, for removal of acid alkyl sulfates, dialkyl sulfates and other reactive and corrosive species, prior to flashing said hydrocarbon effluent.
Processes for alkylating low molecular weight isoparaffin hydrocarbons, e.g. isobutane, with low molecular weight olefins, e.g. propylenes and/or butylenes, in the presence of alkylation catalyst comprising sulfuric acid are well known and widely practiced on a commercial scale. Alkylation catalysts for such processes, of interest in the present invention, comprise sulfuric acid. The sulfuric acid may be present in combination with other catalysts, such as fluorosulfonic acid and may be employed with surface active alkylation reaction promoters. In such alkylation reactions, sulfuric acid reacts, in side reactions, with hydrocarbons present to form by product dialkyl sulfates, acid alkyl sulfates, and acid oils. Acid oils are high molecular weight oils containing substantial amounts of sulfur and oxygen. The major portion of such by-products remain in the acid catalyst phase upon separation of an alkylation reaction zone effluent into a hydrocarbon effluent phase and a catalyst phase. However, a substantial portion of such by-products may enter the hydrocarbon effluent phase, particularly dialkyl sulfates and to a lesser extent, acid alkyl sulfates.
In commercial alkylation processes, alkylation reaction hydrocarbon effluent is subjected to fractional distillation for recovery of unreacted isoparaffin and alkylated hydrocarbon product. The unreacted isoparaffin is commonly recycled to the alkylation reaction zone for maintaining the ratio of isoparaffin to olefin reactant above about 2:1. Such fractional distillation is generally accomplished in several fractional distillation columns equipped with reboilers. The by-products, e.g. dialkyl sulfates and acid alkyl sulfates, present in such hydrocarbon effluent are corrosive or breakdown into compounds which are corrosive, under conditions of temperature in such reboilers. Consequently, common practice is to treat hydrocarbon effluent, prior to charge into fractional distillation columns, for removal of corrosive materials.
Hydrocarbon effluent may be treated with caustic materials, such as aqueous caustic solutions, which react with corrosive materials such as acid alkyl sulfates. Alternatively, hydrocarbon effluent may be treated with adsorbents such as bauxite, which adsorb polar compounds which include neutral alkyl sulfates (dialkyl sulfates) as well as acid alkyl sulfates. Bauxite treating of alkylation reaction hydrocarbon effluent has the advantage of removing most polar compounds, and eliminates water carryover with isoparaffin recycle into the alkylation reaction zone, but produces a large amount of spent bauxite for disposal. Caustic treating has the advantage of employing a liquid solution, such that spent liquor may be treated by common techniques and eventually be disposed of. Over the years, caustic treating has substantially replaced bauxite treating for removal of corrosive compounds from alkylation reaction hydrocarbon effluent.
Alkylation reaction processes employing effluent refrigeration are processes wherein an emulsion of acid catalyst and hydrocarbon reactants are contacted with mixing at relatively low temperatures (about -20.degree. to 100.degree. F. for processes employing sulfuric acid catalyst) in an alkylation reaction zone; wherein emulsion effluent from said alkylation reaction zone is separated into a hydrocarbon phase and a catalyst phase, wherein the separated hydrocarbon phase is flashed at reduced pressure while in indirect heat exchange contact with emulsion in said alkylation reaction zone for refrigeration of said emulsion, wherein flashed hydrocarbon vapor is fractionated in a first fractionation zone for recovery of isoparaffin for recycle, and wherein unflashed hydrocarbon liquid is fractionated in a second fractionation zone for recovery of isoparaffin for recycle and alkylated hydrocarbon product.
In effluent refrigerated alkylation processes, both the flashed hydrocarbon vapor and unflashed hydrocarbon liquid tend to contain polar compounds, such as dialkyl sulfates and acid alkyl sulfates which are corrosive, or which break-down into corrosive compounds, in the reboilers of the fractionation zones. According to common practice, flashed vapors, after heat exchange, are condensed, scrubbed of reactive compounds in a first aqueous caustic treater, and the caustic scrubbed condensate is fractionated in a first fractionation zone for recovery of isoparaffin for recycle to the alkylation reaction. Unflashed hydrocarbon liquid, after heat exchange, is scrubbed of reactive compounds in a second aqueous caustic treater, and fractionated in a second fractionation zone for recovery of additional isoparaffin for recycle and alkylated hydrocarbon product. Such caustic scrubbing results in water (and sometimes caustic) entering the alkylation process with recycle isobutane. Such water increases acid consumption in the alkylation process. Additionally, caustic scrubbing does not remove dialkyl sulfates, which tend to decompose into acidic materials in the presence of water under conditions of elevated temperature in fractionation zone reboilers.
The two caustic scrubbers could be replaced with two bauxite, or other adsorbent, treaters, thereby eliminating water carry-over problems and removing dialkyl sulfates which decompose into corrosive compounds in fractionator reboilers. This however is not a common practice.